Remove the NullGL interface (and associated test context)

Bug: skia:
Change-Id: Ie3c9ee39fc1e0a4406de085c60d8433ffb4419df
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/203708
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
15 files changed